Two-component systems such as two-component adhesives require to be stored in two containers so that the two components are kept separate from one another. The two components are then dispensed in appropriate proportions, and mixed, and begin to cure immediately.
Such adhesives are used chiefly in the civil construction industry to grout steel reinforcing bar and anchor bolting rod into existing concrete. The severe service conditions of steel anchorages into concrete and the high potential risk to life in the case of civil structure failure, places high demands on the anchoring adhesive. Field applications of grouting adhesive must necessarily be "fool proof" and must, for all practical purposes, work every time.
In a typical two-component epoxy comprised of an epoxy paste and a curing agent paste, which chemically react over a period of time after contact, forming a finished plastic with very dense molecular linkages between the epoxy and curing agent, and in which, these linkages are three-dimensional. It must be understood that the relationship of the curing agent paste to that of the epoxy is not catalytic, or initiating in any significant way. The principal reaction taking place during cure is a nucleophilic substitution, resulting in a one to one correspondence of curing agent reactive sites to epoxy reactive sites. It is critical to the strength of the cured epoxy that the components are mixed in the proper ratio, and that they are very well mixed.
A crude solution in the past was simply to have two separate tubular containers, and squeeze appropriate equal quantities from each container. An improvement on this expedient has been to provide as it were a double-barrelled container. In this arrangement, two containers were formed as an integral structure side by side. Two parallel nozzles were provided at one end and two pistons were provided which could be forced through the two containers simultaneously.
This was somewhat of an improvement. However, it still resulted in the depositing of two separate quantities of the two components in physically spaced-apart locations. The two quantities of components then had to be mixed by hand.
One of the disadvantages of this system is that the mixing of the two components after the ejection from the containers was messy, and caused waste. In the particular application of two-component adhesives in the securing of fastening systems in bore holes, it is highly desirable to provide a dispenser cartridge for the two-component systems in which the two components are mixed and ejected as a single combined stream, for a single nozzle, which can thus be deposited directly from the nozzle into the interior of the bore hole.
One of the problems is that such paste adhesives are formulated by the addition of solid powders to the resin base and batched under high shear dispresion (a process in which a high rpm blade forces both turbulence of the slurry, and the high shear collision of solid particles into each other at differing speeds, refining, milling and wetting the solid components into the resin).
Air is entrained into the mixture during this process, which results in the finished paste being somewhat compressible. The amount of air entrained in the batch is a complex function of batching conditions and chemical nature of the resins, and the solids being added. Differing air entrainment in the epoxy and amine curing agent pastes results in unbalanced compressibilities of the two components.
When flow is initiated from the rear of the cartridge and the materials are of unbalanced compressibility, both components will come under pressure within their respective chambers, converting the pressure in part ot volumetric flow of the material, and in part to a volume change of the product due to its compressibility. The least compressible paste will leave the cartridge in higher relative amounts at first, and the more compressible component will compress to a higher degree at first, with lower relative volumetric flow. The resulting initial surge from unbalanced paste components will not hold to the mix ratio set forth by the ratio of cross-sectional areas of the two cartridge chambers.
Under continuous flow conditions, the paste components in the effluent will approach the design mix ratio of the system, as the components achieve a dynamically balanced compressibility at the expense of initial mix ratio. The off-ratio surge material will precede the properly proportioned material out of the nozzle.
The converse situation results a flow shut-down, when the flow is discontinued by terminating pressure at the rear of the cartridge. After termination of pressure the compressed materials relax, i.e. expand, and this causes material to continue to flow into the cartidge nozzle. The material will be off-ratio in accordance with the difference in compressibility of the contained materials. Each time flow is initiated, the material will surge high in the less compressible component. Each time flow is terminated, a relaxation of the more compressed material will produce a volume of material which will be off-ratio. This phenomenon presents great concern in achieving successful cured properties of the epoxy under field application.
The volume of off-ratio surge material dispensed at the initial flow and shut down sequences is a function of the degree of compressibility imbalance and also a function of the column length of imbalanced pastes being compressed. A longer column will accentuate the imbalance, resulting in larger volumes of off ratio material. This characteristic limits the workably safe length of cartridges.
One proposal is shown in U.S. Pat. No. 4,366,919. In this arrangement two cylindrical containers are provided - one being a small cylinder, and the other being a larger cylinder, and the smaller cylinder being located within the larger cylinder. A cylindrical boss is provided on the ejection end of the cartridge which is offset to one side of the cartridge. Openings are formed from the larger and the smaller cylinders which communicate with the boss. An ejection nozzle is attached to the boss, for mixing and depositing the material. This offset arrangement makes the cartridge clumsy and inconvenient to use. The force required to eject the contents of the two containers, is applied along the central axis of the two containers. However, since the dispensing nozzle is offset towards the periphery of the two containers, the containers must be firmly held, while the dispensing force is applied. If the container is not firmly held it will slip or twist, and the placing of the nozzle will be inaccurate. Manually operated tools similar to caulking gums have been manufactured but are unsatisfactory. If the nozzle were located centrally, it wuld overcome those disadvantages.
Proposals have been made for cartridge type containers of the co-axial design, in which the dispensing nozzle is located centrally, along the central axis. However, there are certain problems in providing passageways for the ejection of both components, when using a central nozzle. Proposals that have been made in the past have not been completely satisfactory. Typically, proposals of this type have involved substantial waste space at the dispensing end of the container. The two components within the waste space could never be fully ejected, leading to wastage of expensive material.
A further proposal is shown in U.S. Pat. No. 4,846,373. This proposal is similar to that shown in U.S. Pat. No. 4,366,919, except that a valve is incorporated in the boss. In this way, it is hoped that the off ratio flow after termination of the ejection pressure can be prevented. However, this proposal fails to explain how the initial off ratio flow which occurs on startup of the ejection pressure, can be controlled. A more fundamental problem with this proposal is that even with the addition of a valve, the off ratio flow at termination of pressure, will not be completely controlled. Obviously, since the device is being operated by one man, with only two hands, he will have difficulty in simultaneously terminating pressure and, at the same time, operating the valve, while all of the time holding the cartridge in position. Thus, even with this system, there will still be some degree of off ratio flow at termination of pressure, before the valve can be operated.
This may not seem like a significant problem. However, this mixing and ejection nozzle are located downstream of the valve. The mixing and ejection nozzle contain a quantity of adhesive which is relatively significant in relation to the total amount of adhesive to be deposited in any given bore hole. Thus, assuming a cartridge is to be used for depositing quantities of adhesive in a series of bore holes, then each time the nozzle is inserted in a bore hole, there will be an initial quantity of adhesive ejected into the bore hole, which is off ratio, resulting from the off ratio flow at the termination of the previous ejection, followed by a further quantity of adhesive which is off ratio due to the initial off ratio surge due to startup of the ejection pressure. This combination of the two quantities of off ratio flow amount to a very significant fraction of the total of adhesive deposited in any one bore hole. As a result, there will be a significant variation in the strength of the cured adhesive, leading to unreliable results.
A further problem arises in the filling of such cartridges with the two components. The practice is to fill the two chambers of the cartridge through the two ejection nozzles. While this is being done, the two pistons are located at the extreme rear end of the cartridge. Obviously, the air within the cartridge must be released as the material flows in. In the past this has been achieved by simply inserting a pin or needle along one side of each piston to create a small air gap.
This however does not always result in the ejection of 100 percent of the air. This is partly because of the design of the pistons, which incorporate a U-shaped profile along the leading face in order to provide positive ejection of the material. As a result, even when the air is removed by the expedient described above, some air still remains entrapped in this U-shaped profile.
It is, therefore, considered desirable to provide a cartridge containing two components in respective co-axial chambers, with a dispensing nozzle aligned along the central axis of the two containers, and in which the problem of off ratio flows both at startup and termination of ejection pressure, are minimized.